1. Field of the Invention
This invention relates to an ammonia utilization process which substantially reduces by-product ammonia gas present in product gas from gasification of organic carbonaceous materials and concomitantly enhances the hydrogen content of the product gas by recycling sour water stripper off-gas comprising substantial quantities of ammonia gas to the primary hydrogen forming carbonaceous material gasifier wherein the ammonia is converted to nitrogen and hydrogen.
2. Description of the Prior Art
In many energy conversion systems, particularly those in which carbonaceous materials, such as coal, oil shale, peat, or the like, are converted to product gases, undesirable by-product gases are formed and released. The composition and quantity of such undesirable by-product gases depends upon the process and the composition of the carbonaceous material converted. Undesirable by-product gases typically comprise sulfur compounds including sulfur oxides and hydrogen sulfide, ammonia, carbon dioxide and phenols. High levels of toxic by-products preclude the release of these by-product gases into the atmosphere without extensive treatment and purification. Subsequent treatment of by-product gases is expensive, and significantly reduces the overall efficiency of the energy conversion system.
Product gases discharged from a hydrogen forming carbonaceous material gasifier undergo various cooling and scrubbing processes to separate purified product gas comprising primarily hydrocarbons and hydrogen. It is customary to water scrub the product gases to remove ammonia prior to acid gas cleaning. Undesirable by-product gases, including sour gases, may be dissolved in an aqueous scrubbing agent and thereby separated from desirable product gas. The by-product gases dissolved in aqueous scrubbing agent, referred to as sour water, may then be stripped from the water as gases. It is this mixture of by-product gases which is referred to as sour water stripper off-gas. Many of the prior art energy conversion systems attempt to recover ammonia from the sour water stripper off-gas. Ammonia recovery from off-gas is expensive and a very energy consumptive process, and, in most instances, the quantity of ammonia recovered simply does not justify the energy consumption, particularly in smaller scale systems, and when a relatively low fraction of nitrogen in the substrate material was converted to ammonia.
In many energy conversion systems, by-product ammonia in the gaseous effluent is reduced by injecting the sour water stripper off-gas, comprising primarily ammonia, hydrogen sulfide, sulfur oxides and carbon dioxide, into the burner section of a sulfur plant, such as a Claus sulfur recovery plant. In this system, some hydrogen sulfide and sulfur oxides are converted to desirable elemental sulfur product, and ammonia is destroyed. Injection of sour water stripper off-gas into a Claus sulfur burner may be convenient, but it also suffers serious drawbacks. By-product ammonia in the sour water stripper off-gas represents a significant oxygen demand in the sulfur plant, thereby causing air/sulfur ratio regulation problems, and causing heat release in the system. Additionally, unburned ammonia may foul downstream catalyst beds, while overburned ammonia, which forms nitric oxide, nitrates and nitrites, may also cause serious operating problems. Furthermore, by-product carbon dioxide in sour water stripper off-gas dilutes the sulfur content of the Claus plant feed gas, impairs sulfur recovery, and reduces the efficiency of downstream tail gas treatment.
U.S. Pat. No. 4,143,122 discloses the conversion of by-product gases comprising sulfur-containing and nitrogen-containing compounds in a sulfur producing plant with its tail gases being fed to a catalytic ammonia decomposition plant. In this process, ammonia is decomposed to form nitrogen and active hydrogen. Active hydrogen combines with sulfur-containing compounds to form hydrogen sulfide which may be separated and recycled to the sulfur producing plant, while the remaining hydrogen-containing gas is suitable for use as fuel for undergrate firing.
U.S. Pat. No. 3,773,912 teaches the catalytic conversion of ammonia to nitrogen and hydrogen before a gas stream comprising hydrogen sulfide and ammonia undergo a Claus type sulfur recovery process.
U.S. Pat. No. 4,233,275 teaches purification of a coal gas containing hydrogen sulfide and ammonia wherein high temperature, high pressure, raw coal gas is cooled and expanded for ammonia decomposition and the expanded coal gas heated for further ammonia decomposition.